Generally, it is quite important for a scanning electron microscope or other similar apparatus using an electron beam to correct the astigmatism of the electron optical system, for focusing the electron beam impinging upon a sepcimen to a narrow beam. U.S. Pat. No. 4,214,163 discloses a method of automatically correcting the astigmatism of a scanning electron microscope. In brief, this known method comprises the steps of: (a) making the correcting currents used in the stigmator equal to zero; (b) varying the lens current without altering the astigmatism produced by the lenses; (c) finding the value of lens current at which the electron beam upon the specimen assumes the shape of a circle of least confusion; (d) maintaining the lens current at the found value; and (e) independently and automatically adjusting the correcting currents fed to X-axis and Y-axis coils of the xy stigmator so as to minimize the size of the circle of least confusion.
In the step (c) of the known correcting method, the electron beam upon the specimen surface is shaped into a circle of least confusion and, therefore, it is necessary to vary the lens current at minute steps over a wide range. However, it is difficult to vary the lens current at a high speed, because the inductance of the lens coils is large. Consequently, it takes a long time to carry out the step (c) above. Hence, it is inevitable that a long time is required to perform this series of operations.